For these years, an analyzer based on supersonic jet resonance enhanced multi-photon ionization process (Jet-REMPI method) has been proposed which enables direct and on-line analysis of dioxins. In the process, carrier gas containing sample molecules such as dioxins is ejected in pulse mode from an ejecting device provided with a high speed short duration pulse valve into a vacuum vessel, laser beam is irradiated to the carrier gas flow for selective ionization of the sample molecules and ionized sample molecules are detected for analysis. In identification of sample molecules, mass to charge ratio (m/z) can be used for congeners and resonance wavelength can be used for isomers.
In the case of this system, the biggest technical problem is at what position of a distance from the nozzle of the high speed pulse valve the laser beam should be irradiated with the optimum effect to the carrier gas flow. JPOH 8-222181 discloses a view that the optimum position falls in the region where the carrier gas flow transitions from a continuous flow to a molecular flow. That is, the optimum position for laser beam irradiation, i.e. the ionization zone, is located near an interface between the continuous flow zone formed by expansion in vacuum of carrier gas and the molecular flow zone. From the viewpoint of gaseous molecule kinetics, the distance X of the ionization zone from the nozzle outlet opening is defined as follows;0.5XT<X<3XT wherein XT is a distance from the nozzle to the interface between the continuous flow zone and the molecular flow zone.
In order to perform detection and analysis by Jet-REMPI method of dioxins sample molecules higher than tetrachloride, it is necessary to irradiate laser beam of a pulse width of pico-second and femto-second. This is because dioxin type sample molecules have heavy atom effect by which their excitation lifetime becomes shorter in proportion to the number of chlorine atoms.
Although it has been possible to detect dioxins through irradiation of laser beam of the above-described pulse width to sample molecules, there has been no report of success in their quantification and identification.
Separately from such a process, there is a irradiation process in which sample molecules are irradiated with laser beam of a nano-second pulse width having a photon energy equal to or higher than a energy between ionization potential and excitation triplet state in order to ionize dioxins in a longer lifetime excitation triplet state which transitioned from short excitation lifetime singlet state. Even in the case of this system, however, there has been no report of success in identification and quantification.
The identification of sample isomer molecule by the system disclosed in JPOH 8-222181 is carried out with the resonance carrier gas wavelength intrinsic to the sample molecules. This process is based on a premise that vibration and rotation of sample molecules become discrete spectrum as a result of sufficient cooling of the carrier gas ejected from the high speed pulse valve in the ionization zone.
It is reported in Chem. Rev., 87, (1987) 745-760 by John M. Hayes that generation of characteristics equivalent to a non-pulsed constant irradiation within a prescribed period is indispensable for sufficient cooling of the carrier gas flow ejected from the high speed pulse valve. It is additionally reported that formation of the flat-top portion in the pressure-time distribution is also indispensable when the flow formed into the pressure-time distribution of pulsed gas is observed by fast ionization vacuum gauge.
Minimum retention period of the formed flat top portion is also predicated in the above-described report for various kinds of the carrier gas. When the prescribed period is longer than the minimum retention period, sufficiently cooled gas flow can be obtained.
No substantial means for formation of the flat top portion with sufficient retention period, i.e. the construction of the high speed pulse valve and process of travel of the gas flow ejected from the nozzle through vacuum are, however, reported in either said report nor J. Chem. Phys., 79(12), (1983) 6043-6045 by Katherine L. Saenger and John B. Fenn.
Dioxins are low in vapor pressure. In addition to dioxins, there are lots of gases of low vapor pressure such as organic compounds and their derivatives. These substances are in many cases hygroscopic. When these substances are used for the high speed pulse valve, there is a problem of adsorption to metallic walls. In order to prohibit such adsorption, it is indispensable to use the high speed pulse vale after heating to a high temperature. The heating temperature needs to be 200° C. or higher.
Atomic and Molecular Beam Methods, Oxford University Press, (1988) by Giacinto Scoles discloses high speed pulse valves able to generate pulse supersonic molecular beam. Only two real devices, i.e. “Series 9” by General Valve Corp. and “PSV” by R.M. Jordan Corp. are sold on market. The highest heating temperature is 150° C. for the former and 85° C. for the latter. It is not allowed to raise the temperature higher since the result does not suffice the flow condition of the gas ejected from the nozzle.
It is a choke flow condition of the carrier gas ejected into vacuum via the pulse valve (see the above-described earlier reports). According to the choke condition, a gas flow ejected through the nozzle into vacuum saturates at the maximum flow rate, thereby cooling the ejected carrier gas to an ultra-cold level. This condition is not sufficed since the vacuum sealing element of the pulse valve undergoes thermal expansion, the lift of the valve body of the electromagnetic valve is constant, no sufficient gap can be left between the sealing element and the valve body and, as a consequence, the amount of carrier gas flowing into the nozzle decreases.